J. Phys. Chem. 1995,99, 12179-12187
12179
Quenching of PF(blX+) by F2, Cl2, Br2, N02, and N2O. The Energy-Pooling Reaction with NF(blEf) and the Thermochemistry of PF, PCl, and PBr Yao Zhao and D. W . Setser* Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 Received: January 6, 1995; In Final Form: June 6, 1 9 9 9
Reactive quenching of PF(b'Z+) molecules by Fz, Cl2, and Br2 has been studied at 300 K in a flow reactor. The PF(b) molecules were generated by a passing a dilute flow of PF3 in He through a dc discharge. The total removal rate constants for PF(b) by F2, Clz, Brz, and NO2 are 2.1 x lo-", 8.1 x 1O-l2, 2.3 x and 6.8 x lo-" cm3 molecule-' s-', respectively. Quenching by Nz0 is much slower and probably proceeds by an electronic-to-vibrationalenergy transfer mechanism. The reactions of PF(b'Z+) with Clz and Br2 proceed, in part, to give PCl(b'Z+) and PBr(b'Z+), respectively. The large rate constants for F2 and NO2 suggest that the quenching mechanism could be chemical reactions, even though the products have not been identified. The thermochemistry associated with PCl(b'Z+) and PBr(b'Z+) formation shows that Do(P-F) is no more than 6.9 and 18.0 kcal mol-' larger than Do(P-Cl) and Do(P-Br), respectively. Formation of PF(A311) by the energy-pooling reaction of PF(b'Z+) with NF(b'Z+) was observed. The estimated PF(b) and NF(b) concentrations indicate that the PF(A) formation rate constant is large and this energy-pooling reaction seems to be efficient.
1. Introduction
PF(b'C+)
The metastable a'A and b'Z+ singlet states of the monohalides of the group VA elements, such as NF,PF, NC1, and PCl, are of interest because these molecules, which are isovalent with 02, are potential candidates for chemical energy storage systems.'-5 A systematic study of the reactions of the PF(X3Z-, a'A, b ' F , A3n, and d ' n ) states has been undertaken in our laboratory to explore the chemistry of the PF ~ y s t e m . ~In - ~a separate paper,' we have reported the rate constants for the quenching of PF(b'Z+) by 38 molecules. For the majority of molecules tested, the dominant quenching mechanism is electronic-to-vibrational energy transfer, and the dependence of the rate constants on properties of the reagents can be qualitatively explained by the model proposed by Schmidte8 The present work shows that quenching of PF(b'F) by the diatomic halogen molecules occurs by chemical reaction. Previous studies strongly indicated that the quenching of NF(b) and NCl(b) by diatomic halogens also is due to chemical reaction, but the reaction products were not identified?,l0 In the present work, PCl(b'Z+) and PBr(b'Z+) were identified as the products of the PF(b'F) X2 (X = C1, Br) reactions from their ( b ' p X3Z-) emission spectra, and the branching fractions for the formation of PCl(b'F) and PBr(b'F) were estimated. The thermochemistry associated with the formation of PCl(b'P) and PBr(b'F) will be discussed and used to narrow the large uncertainties for the heats of formation for PCl and PBr. The PO(B2Z+ X2H) emission was detected when NO2 was added to the flow reactor, but analysis of the time profile of the emission showed that PO(BzZ+)was generated by the reaction of NO2 with a species other than P F ( b ' 9 ) . The mechanism for quenching of PF(b) by NO2 could be either excitation-transfer or chemical reaction. The quenching rate of PF(b) by N20 is much slower than by NQ2. One of our objectives in developing a source for PF(b'Z+ and a' A) molecules was to study energy-pooling reactions between singlet metastable states of NF, PF, 02, etc. In the present work, one of the energy-pooling schemes,
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Abstract published in Advance ACS Abstracts, July 15, 1995.
+ NF(b'Z+) - PF(A311) + NF(X3Z-) LW", = -2930 cm-' (la)
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(1b)
was confirmed by observing the PF(A-X) emission. The rate constant for reaction l a was estimated from observation of the relative emission intensities of reactants and products. Reaction l b could not be studied, but the large value for kl, suggests a high branching fraction for the energy-pooling step. The verification of this energy-pooling reaction offers encouragement for studies of similar processes with other pairs of b ' F state molecules.
2. Experimental Techniques The experiment technique was the same as that used previously for studies of the PF(b) quenching reaction^.^ The PF(b) molecules were generated by passing a He flow containing a small amount of PF3 through a dc discharge that was connected to a fast-flow reactor coated with halocarbon wax. The wax coating is required to prevent quenching of PF(b) on the reactor walls. The discharge consisted of a rolled Ta foil cathode and a pin anode in a 2.2 cm diameter Pyrex tube. The load resistor for the discharge was 9 kS2 and the typical applied voltage was 400 V. The PF3 was added to the carrier gas 10 cm before the discharge; a stainless steel needle valve was used to control the flow. The typical PF3 concentration added through the discharge was -5 x 1Ol2 molecule ~ m - this ~ ; concentration was needed to remove all He(23S) before the flow reached the reagent inlet, which was -17 cm downstream in the main reactor. The concentration of PF(b) in the main flow reactor was previously estimated' to be -2 x lo8 molecule cm-3 from comparing the PF(b-X) and 0 NO emission intensities. Subsequent comparison with the NF(b) concentration generated by the A I ~ ~ P o , z N) F 2 reaction suggests that this concentration is a lower limit, vide infra. Reagents were metered into the gas flow through a perforated glass loop injector placed -1 cm downstream from the PF(b) entrance for the quenching experiments. The He pressure in the flow reactor was 2 Torr.
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0022-3654/95/2099-12179$09.00/0 0 1995 American Chemical Society
12180 J. Phys. Chem., Vol. 99, No. 32, 1995
Zhao and Setser
The flow speed was -60 m s-l, corresponding to a time scale of 0.17 ms cm-' for plug flow profile. A flow reactor with two discharge sources was required for the study of the energy-pooling reaction between PF(b) and NF(b). The discharge source for PF(b) was the same one as that mentioned above. The second one, consisting of two rolled Ta foil electrodes, was used to produce NF(b) by passing a NFd Ar mixture through the discharge. The NF2 was generated by flowing N2F4 through a section of flow line heated to 500 K; the heated N2F4 was mixed with Ar immediately before the discharge zone. The typical N2F4 concentration was 4 x 10l2 molecule ~ m - ~Passing . N F 2 through the dc discharge generates the highest known concentration of NF(b) in a laboratory flow reactor.'' To ensure proper mixing and to minimize loss of NF(b) and PF(b) concentrations from wall quenching, the PF(b)/ He and NF(b)/Ar flows were added through concentric inlets into the main flow reactor; the PF(b) flow entered through the inner tube, and the NF(b) entered from the jacket encircling the inlet used for PF(b). The ratio of He:Ar was 3: 1 for a total pressure of 2.4 Torr in the main reactor. The emission spectra were viewed through a quartz window placed -5 cm downstream from the mixing inlets. A photomultiplier tube (RCA C31034) attached to a 0.5 m monochromator (Minuteman) was used to monitor emission spectra from the reactor. A grating blazed at 500 nm was used to observe the PF(b-X) emission at 748 nm for the quenching rate measurements and for the observation of the formation of PCl(b) and PBr(b). A grating blazed at 300 nm was used for the energy-pooling experiments, which required simultaneous recording of the NF(b-X), PF(b-X), and PF(A-X) emission spectra. The response function of the monochromator and photomultiplier (PMT) was calibrated with standard quartz-12 and deuterium lamps. The signal from the PMT was processed by an amplifier/discriminator (EG&G 1120) and a photon counter (SSR 1105) and then sent to a personal computer for storage and subsequent analysis. Tank grade He carrier gas was purified by passage through three molecular sieve traps cooled by liquid-Nz and pumped through the flow reactor by a rotatory pump and a blower. The molecular sieve traps for Ar were cooled by a solid-Codacetone mixture. The PF3 sample (Ozark-Mahoning) was degassed at liquid-N2 temperature; the sample was then loaded by passing the gas through a solid-C02/acetone slurry-cooled trap before entering the Pyrex glass storage reservoir. The N2F4 gas was purified by pumping the sample from the tank through a liquidN2 trap to remove N2 and other uncondensible impurities; the condensed N2F4 subsequently was warmed to solid-C02/acetone temperature, and the gas was stored in a Pyrex glass bulb as a 25% mixture in Ar. The N2F4 in the tank was found to contain about 25% N2. Removing N2 from the N2F4 sample was necessary since emission generated by the N2/Ar discharge would interfere with the PF(A) emission spectrum in the 300400 nm region. The Cl2 sample (Matheson) was degassed by freeze/pump/thaw cycles under liquid-N2 temperature. The Br2 (Fisher) and NO2 samples (Matheson) were degassed by pumping the sample in a solidCO2/acetone cold trap and then expanding the sample to a reservoir. The FZgas (Spectra Gases Inc., 20% in He) was taken directly from the tank without fuxther purification. The loss of F2 from reaction with glass was minimized by connecting the tank to the controlling needle valve by a stainless steel line; a polyethylene line was used to connect the needle valve to the inlet of the flow reactor.
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3. Experimental Results A. Quenching Rate Constants. The PF(b,v' = 0 X, v" = 0) emis'sion intensity at 748 nm was monitored at a fixed
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molecule cm-'1 Figure 1. Semilogarithm plots of the PF(b-X) emission intensity vs reagent concentration. The slope of the lines is proportional to the quenching rate constant for each reagent. The reaction times were 6.7 ms for Br2(0), 8.0 ms for N 0 2 ( ~ and ) F2(A), and 10 ms for C12(0). (1012
distance (therefore, fixed decay time) in the reactor as a function of added reagent concentration [Q]. According to eq 2,
~~([PF(~)~[QI/[PF(B)IIQ~=,) = -~Q[Q]A~
(2)
which follows from a pseudo fiist-order rate e q ~ a t i o nthe ; ~ slope of the semilogarithm plot of the relative intensity vs [Q] gives the quenching rate constant, if the reaction time is known. Figure 1 presents the semilogarithm plots of the relative PF(b) emission intensities vs the F2, Cl2, Brz, and NO2 concentrations. The rate constants are (2.1 f 0.6) x lo-" for F2, (8.1 f 0.8) x for C4, (2.3 f 0.2) x for Br2, and (6.8 f 0.7) x lo-" for N02, all in cm3 molecule-' s-I, as obtained from the slopes of the respective plots. Two separate measurements were done for the rate constants for F2 and Cl2, and the results were virtually identical. The 10% uncertainties assigned to the rate constants reflect the combined uncertainties in concentration measurements and flow speed calibration. The rate constant for F2 should be considered as a lower limit because of possible removal of [F2] by reaction with glass walls of the reactor. The reaction with N20 was also tested, but no reduction in [PF(b)] was found for [N20] = 2 x lOI4 molecule ~ m - ~Thus, . the upper limit to the rate constant for N20 was estimated as 5: 1 x cm3 molecule-' s-l. The rate constants are compared with those for NF(b) and NCl(b) in Table 1. B. Reactions of PF(b) with Fz, Clz, and Brz. The rate constants for F2, C12, and Br2 are large, and reactive quenching mechanisms were suspected rather than a physical electronicvibrational (E-V) transfer process, which would have k~ values in the 10-14-10-'3 cm3 molecule-' s-l range because large changes in vibrational quantum numbers would be required due to the low vibrational frequencies of C12 and Br2. This expectation was confirmed by observing the PCl(b-X) and
J. Phys. Chem., Vol. 99, No. 32, 1995 12181 2.0
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Figure 2. Emission spectra of PF(b,v’ = 0-X,v” = 0) and PCl(b,v’ = 0-X,v” = 0) following the addition of Cl2 to the flow reactor after a reaction time of -2 ms. The spectra have not been corrected for spectral response; the PCl(b-X) band should be multiplied by 1.7 to be on the same basis as the PF(b-X) intensity. The concentrations of PF3 and C12 were 4.5 x IO1* and 9.5 x 10l2 molecule ~ m - ~ , respectively.
PBr(b-X) emissions at 826 and 846 nm,5 respectively, upon admission of Cl2 or Br2 to the reactor, as shown in Figures 2 and 3. No new emissions were observed in the 300-900 nm region when FZ was added to the reactor. Even though the products from PF(b) FZreaction could not be detected, the large rate constant indicates that the mechanism is reactive quenching, probably with formation of PFz F. Typical time profiles of the PCl(b-X) and PBr(b-X) emission intensities are presented in Figures 4 and 5 . These data were obtained by moving the monochromator along the reactor for fixed [Clz] and [Br2]. The intensity-time profiles of PCl(b) and PBr(b) were satisfactorily fitted to a sum of two exponential terms.
Figure 3. Emission spectra of PF(b,v’ = 0-X,v” = 0) and PBr(b,v’ = 0-X,v” = 0) following the addition of Br2 to the flow reactor after a reaction time of -1 ms. The spectra have not been corrected for spectral response; the PBr(b-X) band should be multiplied by 2 to be on the same basis as the PF(b-X) intensity. The concentrationd of PF3 and Br2 were 4.5 x 10l2 and 8.6 x 10” molecule cm-3 respectively. 2000
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Z(PCl* or PBr*) = A[exp(-k,t)
- exp(-k,t)]
(3)
where A, k d , and kr are the amplitude, decay constant, and rise constant, respectively. The first-order decay constant for [PCl(b)] is 274 s-l, which is close to the [PF(b)] decay constant of 216 s-’ measured in Figure 4 under the same experimental conditions. The [PBr(b)] decay constant of 388 s-I is also similar to that of [PF(b)], 343 s-l, for the same conditions. Since the decay rates of the [PCl(b)] and the [PBr(b)] closely follow the decay rate of the [PF(b)], both species must have been formed by reactions of PF(b) with Cl2 or Br2 with the formation reaction being the rate-limiting step. To exclude the possibility that PCl(b) or PBr(b) could be formed by reactions involving metastable P(*D or 2P) atoms, which might be generated by the discharge, CzH2 was added in some experiments after the discharge and before the admission of quenching reagents. The C2H2 concentration should remove
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Reaction time (ms) Figure 4. Time profile of the PCl(b) emission produced by the PF(b) Cl2 reaction. The dots are experimental data points, and the solid line is a least-squares fit to eq 3 in the text.
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P* by addition to the triple bond without a significant reduction of the [PX(b)]. The addition of C2H2 only slightly altered the
Zhao and Setser
12182 J. Phys. Chem., Vol. 99, No. 32, 1995
TABLE 1. Quenching Rate Constants (lo-" cm3 molecule-' s-l) for PF(b) reagent PF(b)" NF(b)b NCl(b) F2 2.1 f 0.6 0.4 f 0.08 0.02' Ch 0.8 f 0.08 1.6 f 0.2 [0.0017,' 0.0007d] Br2 23.0 f 2.0 14.1 f 0.2 NO2 6.8 f 0.7 1.4 f 0.2 N20
